The general objective of this proposal is to clarify how sepsis induces immunosuppression by dysregulating myeloid cell development in bone marrow. Sepsis, a highly lethal and common disease, is initiated by an early systemic hyperinflammatory reaction, which rapidly shifts to a late/chronic immunosuppressive state. Because virtually all anti-inflammatory therapies have failed and most deaths now occur during late/chronic sepsis, emerging investigations seek to restore immunocompetence and hasten resolution. However, the poor understanding of how the chronic immunosuppressive phase of sepsis develops hampers these efforts. To address this limitation, we developed a murine model of late/chronic sepsis and reported that: 1) there is massive expansion of immature myeloid-derived suppressor cells (MDSCs); 2) transfer of these MDSCs into mice shortly after induction of sepsis induces pronounced immunosuppression; and 3) transfer of normal bone marrow CD34+ early myeloid progenitors into sepsis animals markedly improves late survival. We now find in preliminary data that microRNA (miR)-21 and miR-181b increase in bone marrow during late sepsis and that reducing their levels by antagomirs reduces MDSCs, rescues immunocompetence and markedly improves survival. In this proposal, we test the hypothesis that miR-21 and miR-181b induced in bone marrow during sepsis combine to promote MDSC generation by arresting myeloid cell development and that reversing the proximal or distal elements of this pathway will improve late sepsis outcomes. We have designed two aims to test this concept. Aim 1 will determine how miR-21 and miR-181b combine to arrest myeloid development during murine sepsis. Aim 2 will determine how miR-21 and miR-181b expression is induced during murine sepsis. In both aims, we will determine whether disrupting these distal and proximal pathways can restore normal myeloid differentiation, reverse immunosuppression, and improve survival. The potential impact of this study is high, because it will identify bone marrow as a target of sepsis-induced immunosuppression, delineate specific mechanistic pathways, and identify new ways to treat chronic sepsis.